The term hybrid vehicle refers to motor vehicles in which at least two drive units are combined, accessing different energy sources to provide the power for the vehicle drive. The properties of an internal combustion engine, which generates kinetic energy by combustion of gasoline or diesel fuels, and an electric machine, which converts electric energy to kinetic energy, are supplemented especially advantageously. Hybrid vehicles today are therefore equipped primarily with a combination of an internal combustion engine and one or more electric machines.
Two different hybrid concepts can be differentiated. With the so-called serial hybrid concept, the vehicle drive is accomplished exclusively via the electric machine, whereas the internal combustion engine generates the electric current via a separate generator for charging an energy storage device that supplies power to the electric machine and/or for direct power supply to the electric machine. On the other hand, parallel hybrid concepts are preferred today, at least in passenger vehicle applications, where the vehicle drive may be formed by the internal combustion engine as well as by the electric machine.
The electric machines used in such parallel concepts can be operated optionally in motor operation or generator operation. For example, the electric machine is supportively connected to the internal combustion engine (booster operation), typically at operating points of higher vehicle loads. Furthermore, it may assume the function of a starter motor for the internal combustion engine. On the other hand, the electric machine is operated primarily as a generator during driving operation using the internal combustion engine, with the electric power of the electric machine generated in this way being used to charge the energy storage device and/or to supply power to a vehicle electric system, for example. In the case of a hybrid concept with power branching using more than one electric machine, generator operation of an electric machine can also be used to supply power to another electric machine. Furthermore, at least a portion of the braking power is usually applied by the electric machine being operated in generator mode (recuperation), whereby a portion of the mechanical energy loss is converted to electric power. In hybrid concepts it is generally an advantage that the electric machines operate with a better efficiency in comparison with conventional claw pole generators.
The individual components of an electric drive unit include the electric machine itself, power electronics, a power inverter, at least one energy storage device (especially a battery or capacitor storage device) and the like. Due to their high converted electric power and the associated heat loss, these components have a very high cooling demand. Above an electric power of 5 kW, the system is usually cooled, typically with water cooling. Since the electric components, especially in the power electronics, are extremely sensitive to temperature, the temperature threshold values allowed here must not be exceeded because otherwise irreversible damage could occur or the lifetime could be shortened significantly. Likewise, a high thermal shock stress has a shortening effect on the lifetime of components. Consequently, a cooling supply with a sufficiently low temperature and also low temperature fluctuations is desirable here.
Today's power electronics designed for hybrid drives typically have water forward-flow temperatures of approximately 60° C. to 80° C. Thus a direct link of these components to the cooling circuit of the internal combustion engine is extremely critical because, depending on the thermostat design, the size of the vehicle radiator and the precise location of the link, coolant temperatures of 80° C. to 120° C. may be reached. Even if the coolant is branched off immediately after it leaves the vehicle radiator, peak temperatures above 80° C. are reached although the average water temperatures are much lower at this point. One problem with a water tap downstream from the vehicle radiator is the great fluctuations in water temperature due to the thermostatic regulation, which depends on the driving profile.
An arrangement of a cooling circuit of a hybrid drive is known from the older patent application DE 10 2005 003 881.6, for example. The cooling circuit described there includes electronic components of the hybrid drive as well as the internal combustion engine-in a parallel circuit. Due to the arrangement of controllable valves at the connecting points of the partial coolant circuit, which includes the electronic components, partial decoupling from the engine cooling circuit is achieved and thus the direct effect of the relatively high coolant temperatures of the engine cooling circuit on the electronic components is reduced.
With supercharged internal combustion engines, i.e., engines in which the combustion air is compressed especially with an exhaust operated charge air compressor, charge air cooling is generally used. To this end, predominately an air-air cooler situated in the area of the front end of the vehicle is used because this type is associated with low costs. One disadvantage of this type of cooler is its limited cooling capacity, the great dependence on vehicle speed, the large amount of space required and the fact that due to the arrangement at a distance from the engine, relatively long lines to and from the charge air cooler must be accommodated, which is a disadvantage with regard to the dynamics and response of the cooling. In this regard, water-operated charge air coolers that allow a high cooling capacity, take up much less space and therefore can usually be installed directly on the engine with short connecting lines are advantageous but they are associated with significantly higher costs. A high cooling performance with very good dynamics can be achieved with this design.